It annoy's the heck out of me when I hear people say that. It's not about the piece of paper. But from a legal standpoint that piece of paper is important.
To me that's an excuse from someone who isn't ready to make a full commitment to you period. My son used to play with this boy that lived by us, his parents weren't married 2 kids and no plans to marry. I didn't question it, his Mom brought up that they'd been together so long that they are common law married. She said she had no problem not having a marriage license, etc, etc. This is until the day he left, then it was we aren't legally married so it's going to be hard to get spousal support, etc, etc. Well he came back so I guess they are common law married again.
This guy didn't need a bright blinking sign over his head to say, I don't want to officially commit to this woman, for me to know, he didn't want to. Sad for her, sad for the kids. They moved I think they are still together, but I don't know.
You don't have to have a giant wedding, just the wanting and willingness to commit to each other. When people it's apiece of paper, it's a slap in the face to me. I've been married 14 years, 2 children, and good times and bad. I take my marriage vows seriously and I can tell from the way my husband behaves he does too.